Update message severity colors, fix navbar double border (#37019)

- Tweak serverity background and border colors
- Use default text color instead of per-severity text colors.
- Replace `saturate` filter with semibold font weight on message headers.
- Fix navbar double border when a notification is present.

Co-authored-by: Claude (Opus 4.6) <noreply@anthropic.com>

web_src/css/modules/message.css

@@ -43,7 +43,7 @@
 
 .ui.message .header {
   color: inherit;
-  filter: saturate(2);
+  font-weight: var(--font-weight-semibold);
 }
 
 .ui.info.message,